Apparatus for melting magnesium



* Patented Sept. 12, 1939 UNITED STATESPATENT OFFICE` APPARATUS Fon MELTING MAGNESIUM Claire S. Harris, Midland, Micl., assigno' to The Dow Chemical Company, Midland, corporation of Michigan- Application June 25, 1938, Serial Noi 215390 and improved apertured cover for a melting pot' from which the molten metal is withdrawn with 10 a ladle from time to time. i

It is generally known that great dimculties are encounteredin melting and casting magnesium and its alloys owing to the ease with which the molten metal is attacked and destroyed by the,

15 oxygen and nitrogen in the air. Theattack upon the molten metal is so rapidthat an open pot of the metal is unworkable unless the surface of the metal is covered with a layer of protective fiux. However, the use of a flux for the purpose is 20 generally undesirable when metal is to be ladled from the pot in frequent successive relatively' small amounts, as in die casting, because it is.

almost impossible to ladle molten metal from the pot without entraining some of the fiux floating 25 upon the surface, unless theflux is swept aside each time the ladle is dipped into the metal, in which case the surface of the molten metalis unduly exposed to the destructive action of air.

Molten metal containing such contaminating ma- -30 terial formed by action of the air is unsuitable for die casting purposes.

It has been proposed to make use of inert gases or substances which generate inert gases over the surface of the readily oxidizable molten metal to 35 protect it against attack by the air. However,

it is necessary to confine such a gas over the metal, as 'by means of a cover over the melting' pot, for otherwise'air'readily reaches the metal 'and attacks it vigorously. Such a covered pot is 40 unsatisfactory for operations wherein relativeiy small amounts of metal are to ,be ladled from the melting pot and transferred to a die casting machine at frequent intervals, because the cover must be removed each time metal is withdrawn- 45 from the pot. To overcome this difliculty it has been recently proposed to provide a cover for the melting pot having an aperture in a substantially vertical plane above the surface of the metal through which the ladle is inserted and with- 50 drawn', while an inert gas is being fed into the pot over the metal, and a stream of hot combustion gas is passed across the aperture in a plane parallel to the plane of the aperture to prevent the entrance of air into the furnace 55 through the aperture.

Howver, I have found that this latter type of cover is unsatisfactory in commercial use because hot air, rising around the furnace as the metal in the pot is heated and maintained in a molten 'state during the ladling operation, fiows directly past the face of the vertical aperture, causing strong convection currents to be setup around the vertical aperture, so that the protective gas is sucked out or drawn from over the molten metal through the top of the vertical aperture. The protective gas fiowing out from the top of the aperture tends to create a condition of lowered pressure or partial vacuum within the pot,

so that a portion of the air stream rising past the vertical aperture is deflected and drawn into' the pot along with some of the combustion gas directed across the aperture. During the ladling operation the gas currents around the aperture are still further disturbed as the ladle is inserted and withdrawn from the pot and more air and hot combustion gas enters the opening and reaches the surface of the metal. Both the a'ir and the hot combustion gas vigorously attack the molten metal, causing pronounced lossof metal as well as the formation of undesirable reaction products which contaminate the metal, rendering it unfit for use.

It is, therefore, an object ofthe invention to provide an apertured cover for a melting pot useful in melting, refining and particularly for protecting magnesium and its alloys while they are being held in readiness for ladling to a die casting machine, of such a nature that air or gases which' destructively attack molten magnesium may be substantially excluded from reaching the 35 surface of the molten metal through the aperture during the operation of melting and ladling molten metal from the pot.

Other further objects will be apparent as the description of the invention proceeds. 40

I have discovered that by using a dome shaped cover of spaced double walled Construction, having an aperture therein in a plane inclined to between about 20 to 40 degrees with the horizontal and located at a point substantially below the top of the dome, melting and ladling Operations can be carried out while a protective gas such as sulfur vapor, generated within the space between the walls of the cover, is directed across the face of the aperture and over the metal without substantially any disturbing convection currents being set up under the cover over the metal or eddying of gases in front of the aperture. My apertured cover makes it possible to hold in molten condition a 'pot of magnesium and its alloys at the casting temperature for relatively long periods of time without material attack upon the metal by the atmosphere.

A preferred method of carrying the invention into effect will now be described with reference to the accompanying drawing wherein:

Figure 1 is 'a front elevation view in section taken along lines I-l in Figure 2 of the apertured cover and melting pot in a furnace setting.

Figure 2 is a top plan view of the cover.

Figure 3 is' an isometric view of the aperture in the cover as seen from above.

In the drawing a circular oil or gas fired furnace setting 4, provided with a burner |3 and fiue !4, contains a suitably shaped melting pot 5 covered by the conical doubled wall apertured cover 6, which forms an abutting rim at 'l sealing the cover 6 to the melting pot 5. An inclined aperture 8 large enough to admit a ladle is shown located in the plane of the conical cover and at a point near the lower rim 'I of the cover 6. The side wall 9 of the aperture 8, formed by the strip connecting and sealing the double walls |5 and !6 of the cover 6 around the aperture 8, is shown with spaced ports or jets |0 located therein, through which sulfur vapor is discharged and directed across the face of the aperture from the space between the walls of the cover while the metal is at heat and during the ladling operation. The cover is also shown with a suitably capped port l I located Conveniently in the outer Wall of the cover '6 opening into the space between the walls of the cover and with a small port |2 located in the inner wall !6 of the cover opening into the space above the molten metal.

In operation', a quantity of sulfur is charged through the capped port I into the space between the walls of the cover 6 wherein it melts forming an annular pool ll. Heat from the metal in the pot causes sulfur vapor to form or generate within the space enclosed by the walls of the cover and, under the moderate pressure developed as the vapor continues to form, it is forced across the aperture from the ports o located in the side walls 9 of aperture 8 and into the space over the molten metal from port l2.

I have found that the slope or angle of the conical cover with the horizontal should be from between about 20 to 40 degrees to overcome the undesirable effects aforementioned of convecton currents forming around the aperture during the operation of the furnace. The aperture should be located at some 'distance below the'upper most portion of the cover and should 'preferably be located near the lower rim of the cover as is illustrated, to provide a space near the top of the cover for the hot gases to collect above the metal.

I have found that the side walls of the aperture preferably should be substantially perpendicular to the plane of the cover walls in order that eddying around the aperture be effectively prevented. Although usually unnecessary, a gas. such as air, may be introduced into the space between the double walls of the cover so as to force the sulfur vapor therefrom, if insuflicient vapor pressure is developed as the metal is melted and maintained in a molten state.

If desired, the small port |2, opening from the inner wall -of the cover into the space over the molten metal, may be omitted and sulfur vapor directed only across the aperture during the melting and ladling operation, although the sulfur vapor forced from this port over the molten metal serves to give additional protection to the molten metal.

Among the advantages of the invention are that the metal is always readily accessible while protected from oxidation or attack by the air, convecton currents do not draw air into the pot over the metal, and an external generator of a protective gas is not required.

Other modes of applying the principle of my invention may be employed instead of those explained, change being made as regards the apparatus herein disclosed, provided the means stated by any of the following claims or the equivalent of such state means be employed.

I therefore particularly point out and distinctly claim as my invention:

1. An apparatus for melting readily oxidizable metals such as magnesium and its alloys comprising a melting pot, a double-walled, domeshaped cover for said pot, the walls thereof being spaced apart to form a closed chamber in which to generate a protective gas, an aperture through both walls of said cover of sufficient size to admit a ladle, said aperture extending from'the edge of the cover to a point substantially below the apex of said cover, a side wall for said aperture joining the inner and outer walls of the cover to separate said gas-generating chamber from the aperture, a series of small ports in said side Wall communicating with said gas-'generating chamber, and a closable loading port in the outer wall of said cover.

2. Apparatus according to claim 1 in which.

the double-walled cover is conical with substantially parallel walls inclined at an angle of from 20 to 40 with the horizontal.

3. Apparatusraccording to claim 1 in which the double-walled cover has a gas port in the inner wall communicating with the space above the melting pot.

CLAIRE S. HARRIS. 

